Although it will become evident to those skilled in the art that the present invention is applicable to a variety of implantable medical devices utilizing pulse generators to stimulate selected body tissue, the invention and its background will be described principally in the context of a specific example of such devices, namely, cardiac pacemakers for providing precisely controlled stimulation pulses to the heart. However, the appended claims are not intended to be limited to any specific example or embodiment described herein.
Pacemaker leads form the electrical connection between the cardiac pacemaker pulse generator and the heart tissue which is to be stimulated. As is well known, the leads connecting such pacemakers with the heart may be used for pacing, or for sensing electrical signals produced by the heart, or for both pacing and sensing in which case a single lead serves as a bidirectional pulse transmission link between the pacemaker and the heart. An endocardial type lead, that is, a lead which is inserted into a vein and guided therethrough into a cavity of the heart, includes at its distal end an electrode designed to contact the endocardium, the tissue lining the inside of the heart. The lead further includes a proximal end having a connector pin adapted to be received by a mating socket in the pacemaker. A flexible, coiled conductor surrounded by an insulating tube or sheath couples the connector pin at the proximal end and the electrode at the distal end.
To prevent displacement or dislodgement of the electrode and to maintain the necessary stable electrical contact between the lead tip and the endocardial tissue, the electrode must be firmly anchored relative to the tissue. To achieve this, the electrode of one known type of lead comprises a pointed helix adapted to be screwed into the heart tissue to be stimulated. Rotational torque applied to the connector pin at the proximal end of the lead is transmitted via the flexible, coiled conductor to the helical electrode which is thereby screwed into the heart tissue. In this fashion, the position of the electrode tip is mechanically stabilized, that is, the tip is positively anchored so as to remain securely in place during the lifetime of the implant. Removal of the screw-in electrode from the endocardium can be effected by counter rotation of the connector pin. Thus, in a rotatable pin, screw-in lead the conductor coil is used not only as an electrical conductor coupling for the connector pin and the helix electrode, but also as a tool for extending or retracting the helix electrode relative to the distal tip of the lead during lead myocardium fixation by rotating the connector pin.
It is desirable to minimize the number of revolutions of the lead connector pin required to fully extend or retract the helix electrode during lead fixation. The number of connector pin turns is a function of several factors:
(a) conductor coil stiffness, with a stiffer coil requiring fewer connector pin turns (a very stiff conductor coil, however, results in a very stiff lead that potentially creates problems such as high chronic pacing threshold or tip myocardium perforation); PA1 (b) friction between the helix electrode and seal, where such a seal is utilized to prevent ingress of bodily fluids; PA1 (c) the length of the lead body (a longer lead requires more turns of the connector pin to advance or retract the helix electrode a given distance); and PA1 (d) friction between the conductor coil and the surrounding insulating sheath or tube.
With respect to factor (d), to minimize frictional resistance between the "torque transfer" conductor coil and the surrounding insulation tubing, polyurethane tubing has been preferred over silicone tubing because the coefficient of friction between conductor coils (such as the multifilar MP35N conductor coil utilized in various screw-in pacemaker leads manufactured by Siemens Pacesetter, Sylmar, Calif., U.S.A.) and polyurethane tubing is less than that between such coils and silicone tubing. However, because the use of polyurethane tubing has several disadvantages, such as stiffness and limited long term biostability, it would be desirable to use silicone tubing instead.
Accordingly, it is an overall object of the present invention to provide a screw-in lead assembly using silicone insulation tubing but in which the friction between the conductor coil and tubing, and hence the torque and number of connector pin turns required to extend or retract the helix electrode, is substantially reduced.